Contamination of domestic water supplies by harmful or objectionable compounds is of increasing concern in areas of the country which rely upon ground water. Ground water source contamination by thousands of abandoned and unattended underground gasoline and oil tanks is a growing national problem. Migration of toxic or hazardous gasoline additives through the soil is particularly troublesome. Risk is also posed by the radioactive gas Radon 222 increasingly discovered in water sources of New England and other regions of the country. Very high levels have been measured in well water supplies at locations in Maine and New Hampshire originating from granitic rock with relatively high uranium content. Pesticide residues are found in wells throughout areas such as the Connecticut River Valley following years of chemical pesticide application. Moreover, groundwater supplies in many areas of the country carry objectionable odors from naturally occurring sulfur compounds.
Community water supplies and water companies are able to correct some of these problems using large-scale treatment equipment, for example, aeration systems. Typically, however, such aeration systems were developed and used for treatment of sewage and sludge. Representative examples of sewage and organic wastewater aeration treatment 35 systems include the system described in the Mallory U.S. Pat. No. 2,138,349 in which a rotating diffuser of radial porous diffusion tubes rotates at the bottom of a large scale tank. Heil et al. in U.S. Pat. No. 4,501,664 describe a method for treating organic waste liquid and activated sludge which includes two successive aeration compartments followed by a separator and clarifier. In the aeration compartments diffuse air bubbles flocculate and saturate the sludge. In the Clough et al. U.S. Pat. No. 4,029,581 inflatable cells disposed at the bottom of a tank or pool are sequentially inflated and deflated for agitating the sewage. Release of air through orifices of the cells provides aeration.
A number of aeration systems have been proposed for domestic water treatment. Schleyer et al. in U.S. Pat. No. 2,872,415 describe the atomization aeration of water in a spinning disk atomizer water treatment apparatus for "complete elimination of objectionable absorbed gases and complete oxidization of oxidizable substances". U.S. Pat. No. 4,371,383 describes a radon removal system for spraying, recycling and respraying water in a reactor tank "to purge the water of radon gas . . . breaking up the water droplets for release of radon gas." In U.S. Pat. No. 4,491,549 jets of gas are injected for bubbling through a tank.
A disadvantage of these proposed prior art systems for aeration purging, oxidizing, and eliminating volatile contaminants from domestic water supplies is that relatively large, separate, and expensive appliances are required to be installed in the home or other building. Furthermore, this prior art is not directed specifically to the problem of aeration purifying a domestic water source contaminated with gasoline and oi1.
One method of treating domestic water supplies to remove gasoline and oil components uses activated charcoa1 or carbon filter systems. Typically, one to three granular activated carbon (GAC) units or beds are used for water treatment to control taste and odor-causing contaminants. Such GAC units are often utilized as a short-term solution for gaso1ine contamination until an alternative supply can be secured. Generally, multiple units or beds are required in series because of the potentia1 for early breakthrough of contaminants in single unit systems and to meet the restrictive breakthrough criteria for drinking water supplies.
While the initia1 capital cost for installation of GAC treatment systems may be comparable to aeration systems, the maintenance of the system and regular bed replacement is costly. Other disadvantages of GAC treatment systems include the potential for biologica1 growth, cost1y sampling and monitoring to determine the presence of harmful breakthrough of contaminants which may occur at levels below taste and odor thresholds, and the extremely high operating cost for wells that contain high concentrations of gasoline. Furthermore, pretreatment of the water supply may be necessary for iron and manganese removal to protect the GAC filters.
The treatment of water supplies for gasoline contamination is particularly problematic because of the hundreds of components contained in gasoline. It is difficu1t to set meaningful breakthrough criteria for GAC treatment systems for all of the components. Only a few of the components in gasoline have EPA guidelines for exposure through drinking water and these guide1ines are controversial. From a practical standpoint, the homeowner requires removal of any residual contaminants in the treated water detectable by odor or taste. As a result, the cost of GAC treatment rises accordingly. Ideally, all volatile contaminants should be removed or stripped from the drinking water to a leve1 below the limits of detection.
Reverse osmosis technology may be used for treating contaminated supplies to drinking water quality and commercial units are available to treat entire household water supplies. However, reverse osmosis systems have four basic problems which make their application unlikely. Such systems are more costly than GAC treatment, use a water volume of at least four times the normal supply, require a higher operating water pressure, and produce a waste stream of three to four volumes per treated volume with a higher contamination level than the raw water.
Municipal-scale treatment of water supplies contaminated with volatile organic compounds (VOC's) includes aeration systems where venting of the stripped VOC's is permitted. The various methods of aeration include diffused aeration, spray aeration, mechanical surface aeration, and packed tower aeration. Packed tower aeration may require an unrealistic tower height for highly contaminated wells and water supplies making diffused aeration the most practical alternative. Lowry et al., "Proceedings of the National Well Water Association Conference", Portland, Me., Ju1y 1985, published Sept. 1985. However, the prior art does not include domestic scale water treatment with continuous staged flow approximating "plug flow" for stripping volatile contaminants from a water supply and restoring potable water quality on demand.